(a). Field of the Invention
The present invention relates in general to a multi-carrier communication system, and more particularly to a method for initiation and stepsize control of a time-domain equalizer in a multi-carrier communication system.
(b). Description of the Prior Arts
Multi-carrier modulation is widely used in communication systems nowadays. FIG. 1 is a block diagram of a communication system 100 using multi-carrier modulation. The communication system 100 employs a set of N-point Inverse Fast Fourier Transform (IFFT) 102 in the transmitter and the Fast Fourier Transform (FFT) 111 in the receiver to transceive data. A channel 106 is divided into N subchannels for transmitting data. The signal transmitted by one subchannel is orthogonal to those transmitted by any other subchannels. Thus, the signal transmitted by the subchannels would not interfere each other, and inter-channel interference (ICI) can be avoided.
The set of N-point data outputted from IFFT 102 is called a symbol. Since the channel impulse response (CIR) is not ideal, that is, the amplitude/frequency response of the channel are not constant across the all used subchannels, then the received signals will different from the transmitted signal, and the signals presented to the QAM decoder 113 will different from the signals outputted from the QAM encoder 101. If the distortion is severe then the data transmitted by one subchannel will affect both the data transmitted by other subchannels (ICI) and the data transmitted by that subchannel in the previous and subsequent symbol periods (inter-symbol interference, ISI). In order to avoid inter-symbol interference (ISI) and ICI, a “cyclic prefix” (CP) is added to each symbol, i.e. the last υ points of each symbol are copied and added in the front of the symbol, as shown in FIG. 2. Therefore, each symbol outputted from adding cyclic prefix circuit 103 includes (N+ν) points. The adding cyclic prefix circuit 103 in the transmitter and the removing cyclic prefix circuit 110 in the receiver of FIG. 1 are used to add and remove cyclic prefixes, respectively.
In the communication system 100 of FIG. 1, if the valid length of channel impulse response (CIR, denoted by h[n]) is shorter than the length of cyclic prefix, then a symbol, after being transmitted in the channel 106 and received by the receiver (i.e. the convolution of the symbol and the CIR, h[n]), would not affect the data of the subsequent symbol received in the subsequent symbol period. However, if the length of CIR is larger than that of the cyclic prefix, then ICI and ISI will occur. Under this circumstance, a time-domain equalizer (TEQ) with an impulse response w[n], as shown in the block 108 of FIG. 1, is necessary for the receiver of the system 100. The TEQ 108 is used to modify the CIR of the communication system 100, such that valid length of the modified CIR (called target impulse response and denoted by b[n]), i.e. the convolution of the CIR h[n] and the TEQ impulse response w[n], is shorter than that of the cyclic prefix, thereby preventing the received data from ICI and ISI.
Since the CIR is different with various transmission channels, the TEQ impulse response needs to be adjusted accordingly. Many adaptive TEQ algorithms are developed in succession. Since these adaptive TEQ algorithms are sensitive to initial values of TEQ impulse response W[n] and target impulse response B[n] in frequency-domain (In the following description, time-domain variables will be referred to by lower-case letters and frequency-domain variables will be referred to by capital letters), the adapting result would be unreliable if the initial values are not properly determined during the TEQ initialization process. The adapting result will fail into the local maxima point.
U.S. Pat. Nos. 5,285,474 and 6,396,886 disclosed the conventional adaptive TEQ algorithms. However, both of these two patents failed to disclose the method to determine the initial values of frequency-domain TEQ impulse response W[n] and target impulse response B[n]. In addition, the coefficient (called stepsize coefficient in this specification) used to stepwise adjust W[n] using frequency-domain Least Mean Square (LMS) will greatly affect the performance of the adaptive TEQ algorithms. If the stepsize coefficient is set too small, the converging speed will be too slow; if too large, then a diverging result will occur frequently. Both cases would degrade the system performance seriously.